1. Field of the Invention
The present invention relates to a cooling apparatus for cooling printing paper after fixing in a thermal fixing type printer, and particularly relates to a paper cooling apparatus in an electrophotographic printer.
2. Description of the Prior Art
In an electrophotographic printer, the printing paper is commonly heated in a thermal fixing process so that toner adhering to the paper is fused and fixed thereon forming characters and figures on the printing paper. The paper carrying the toner image fixed thereon is then folded by and received in a stacker (a paper folding device). If the thermally fixed paper is folded before it is cooled lower than the glass transition point of the toner, the toner is fused at opposing portions of the folded printing paper where characters or figures formed on opposite printing surface portions of the paper are superimposed on each other. Therefore, the toner on one printing surface is peeled off when the printing paper is unfolded. Thus, characters and figures may be sometimes partly removed. This phenomenon is called "toner stick" which may be a serious quality problem in a printer.
In order to prevent such toner stick from occurring, the paper heated in the fixing process can be cooled to lower the paper temperature to a value not higher than the glass transition point of the toner. Although natural cooling or forced cooling by a small-sized fan or blower has been conventionally used to cool printing paper, the paper running speed is so high in printers having a super high printing speed of about 15,000 lines per minute that the time the paper is cooled too short and the paper cannot be sufficiently cooled. One solution is to elongate the paper path system between the fixing device and the stacker and to provide a large-sized fan or blower so as to increase the paper cooling rate. However, these both militate against miniaturization, reduction in power, and reduction in cost of the printer, all of which are desirable.
FIG. 4 shows an example of the paper cooling apparatus according to the prior art described above. Printing paper 1 is heated and fixed by a pre-heater 10 and a heat roll 8 respectively, and forcedly cooled by the cooling air blown from cooling blowers 11. Next, the fixed and cooled paper is folded up by paddles 4, and stacked on a table 6. In the case of a high-speed printer in which the speed of paper passing between the conventional cooling blowers 11 is high, the time during which the paper 1 can receive the cooling air is so short that a sufficient cooling effect cannot be obtained. In order to prevent toner stick from occurring, therefore, it is necessary to provide a large-sized fan or blower corresponding to the paper running speed so that the paper 1 can be cooled sufficiently. In this case, however, the apparatus becomes large in size, and increases in weight, in power consumption, and in manufacturing cost.